Adding a diff comment, then deleting it makes it impossible to add a new comment on the same line

1. Go to a merge request diff
2. Add a line comment
3. Delete it
4. Attempt to re-add a comment on the same line; button no longer shows on hover

You might be thinking "Just edit your existing comment, dummy!", but this happened to me multiple times today during reviews. I ask a question, answer it myself or doubt myself, delete it, then realize I was right and go back to add it. 😆